Tuesday, September 29, 2009

The GFS has slowed down the frontal passage (Fig 1) by about 150 miles and is coming more in line with the NAM and WRF (Fig 2). We still do not see a dry line forming on the GFS solution so we would be looking mainly at a squall line event Thursday evening. The NAM still shows a dryline and develops a secondary surface low, much like the GFS was hinting Monday morning, causing backing winds over north central TX and south central OK (Fig 3). This will aid in low level mesocyclone development which could lead to a tornado or two developing during storm initiation. I still like the northern target better for the possibility of tornadoes but the 12Z NAM has a better potential for tornadoes in the Southern Plains. Looks as though there could be a significant tornado or two late Thursady into Friday in eastern OK and north west AR but the chase terrain here is not the greatest. As of now I will probably head out Thursday after Midday and wait to see what happens.

We are still looking at a threat of severe weather developing Thursday across the Central U.S. but depending on which forecast model you go by, there is a very different result. The image above shows the forecast for Thursday evening at 7PM CDT. Notice the cold front is much farther east than the forecast for the NAM (see below). The scenario forecast by the NAM would be much more conducive for supercell and tornado development Thursday afternoon and evening so this will be the model of choice to discuss, since the GFS shows a fast moving squall line with mainly a gusty winds small hail event.

Looking at the forecast map for the NAM for 0Z Friday ( 7PM CDT Thursday Evening) you see an area of low pressure with a warm front extending east through central Illinois and a cold front draped across central Kansas and northwestern Oklahoma. Out ahead of the front a dryline sets up and would be the focusing mechanism for thunderstorm development if this scenario unfolds. There are two areas that seem favorable for supercell development which could lead to isolated tornadoes late Thursday.

The first area that seems likely a few supercells could develop is along and east of I-35 in central Oklahoma south into North Texas north of Dallas. A large area of low level helicity will combine with moderate instability and moderate CAPE across these areas. 0-1km storm relative helicity are between 100 and 250 from the Dallas Forth Worth area through the Eastern third of Oklahoma (Fig 1). This should provide adequate low level turning of height to help produce low level meso cyclones as thunderstorms develop. In the same area surface based CAPE is on the order of 1500 to 2000 J/kg (Fig 2), not extreme but enough to have robust updraft strength in this area of low level helicity. Finally the surface based lifted index is around -4 in this same area providing moderate levels of instability (Fig 3).

Fig 1Fig 2Fig 3

The second area of supercell development and in my opinion may be the big winner when it comes to tornado development is out ahead of the surface low across southeast Nebraska, southwestern Iowa, eastern Kansas, and northwestern Missouri. An area of higher instability, CAPE and strong low level helicity will combine for the development of scattered supercells along the dryline/cold front. The images below show the surface based lifted index and CAPE for 4PM CDT Thursday Afternoon. LIs approach -5 (Fig 4)with CAPE approaching 2000 J/kg (Fig 5) in southeastern Nebraska. This area will also be experiencing very strong low level relative helicity, approaching 250 (Fig 6). For chasing Thursday I really like the northern option out ahead of the surface low for tornado development.

Fig 4Fig 5Fig 6Again the following forecast is based on the NAM forecast model. I feel the GFS may be bringing this system through too fast as the water vapor imagery shows the 500mb vort max is moving into the Pacific Northwest slower than the GSF advertised on the 0Z run. I will issue another update later this afternoon after analyzing the 12Z model runs.

Monday, September 28, 2009

A quick update to share what the 12Z model runs are showing. The GFS continues to move the cold front rapidly through the Southern Plains. The 12Z run speeds up the frontal passage and puts it near the I-20 corridor by 7PM Thursday evening.(See image below) I feel this may be a little too fast. The instability forecast is also much lower on the 12Z run decreasing the severe threat.

The 12Z NAM is now in the forecast time frame and is showing a much slower and better severe weather set up than the GFS. The NAM is forecasting a dryline to set up Thursday afternoon across central Oklahoma and hangs the actual cold front across northwest Oklahoma Thursday evening. This will allow for plenty of Gulf moisture to move in across the Southern Plains(See image below).

Below is the forecast for CAPE (convective available potential energy) for the NAM Thursday evening. Notice the energy is pooling along the dryline in central Oklahoma.

With the much faster GFS solution CAPE is confined to East Texas and is no where near as much as advertised by the 0Z GFS.(See image below) This solution would still produce storms and with the shear available, we could see a line of storms with gusty winds across the Southern Plains.

Overall I feel the GFS is too fast and the NAM is a bit too slow. Both models do not point to a significant tornado threat as wind shear will be mainly unidirectional. However, if a secondary low develops as the 0Z GFS advertised, we could see an enhanced tornado threat along and just north of the low. This system still bears watching and may be a worthwhile chase for me later this week.

A strong storm system is forecast to move into the middle of the country Thursday and drag a cold front through the Southern Plains. Current forecast models are coming to an agreement on the position and strength of this system. There is a possibility of severe weather along and ahead of the cold front from eastern Nebraska into Texas late Thursday through Friday morning. Right now the main threat with this line appears to be strong winds and hail but, I can’t rule out a possible tornado or two late Thursday, especially near the Oklahoma Texas state line north of Dallas.

Below is the forecast position of the 500mb Vorticity maximum late Thursday by the GFS forecast model. Along with this strong area of vorticity, this image shows diffluent heights across Ozark Plateau and Southern Plains. This will aid in the lift for the formation of strong storms late Thursday.

This storm system is still too far out to show on the short range NAM model run for Thursday evening however, it is showing up Thursday morning on this model and is a very similar position to the long range GFS forecast model. The next two images show the comparison between both models. Not much difference.

There also appears to be plenty of Gulf moisture available for this upcoming storm system. By Thursday morning both forecast models show a large area of dewpoints over 70°F along the Texas Gulf Coast with the GFS showing a larger supply of moisture.

By Thursday evening the GFS is showing plenty of deep moisture with mid 60 dewpoint temperatures being pulled into the storm system across the Southern Plains and 70+ degree dewpoints across southeast Texas.

Now the GFS is hinting at a smaller secondary surface low (shown below) developing somewhere near the Oklahoma Texas state line. This is an important feature because along and to the north of this secondary low, surface winds will have an easterly component, or what we call backing. This will increase the threat of tornadoes with any storm that can develop in this area.

We also see plenty of instability with this system. Surfaced Based lifted index in south central Oklahoma are approaching -8, more than enough for strong to severe storm development. (See image below)

There is a maximum of CAPE (Convective Available Potential Energy) near the maximum of instability in south central Oklahoma.(See image above) This will allow for stronger thunderstorm updrafts aiding in the possibility of large hail. Now, if the best instability and energy develop in the area just northeast of the developing secondary low pressure, the chances of tornadoes will increase, especially during initial thunderstorm development. The GFS is showing this threat with the 0-1km EHI (Energy Helicity Index) of 4 (Values greater than 2 have been associated with significant tornadoes) or greater near Lake Texoma. (Image below)

Other experimental severe weather indices are pointing to a possible severe weather event as well. The two images below the Craven Significant Severe weather index and the Supercell Composite Parameter. The second image is showing a bulls eye near the same area of increased instability and CAPE.

So will we see an outbreak of tornadoes? Well we won’t know for sure until we get closer to the forecast event however, the last image shows a long line of enhanced 700mb vertical motion, or lift. This indicates the storms that form will quickly form into a squall line so the best chance of isolated supercell thunderstorms will be during the early evolution of storms late Thursday afternoon. With the associated shear anticipated Thursday evening, this line of storms will more than likely take on a line echo wave pattern (LEWP). This will allow for a few isolated supercells to be embedded in the squall line increasing the threat for strong winds and an isolated tornado. I will have an update once the NAM forecast model is in the forecast timeframe.

Tuesday, September 22, 2009

The National Weather Service will unveil two high water mark signs commemorating the historic April 1945 flood in Jefferson, Texas, on Tuesday September 22, at 11 a.m. One of the signs will be installed on the front of Henderson’s Auto Parts Store at 218 Polk Street. A second sign will be installed on the Jefferson Jimplecute building at 205 West Austin Street. This historic ceremony will be held at Henderson’s Auto Parts Store, and the public is welcome to attend. Armando Garza, Meteorologist In Charge of the Shreveport Weather Forecast Office said, “The High Water Mark sign at Jefferson is the first such sign in the State of Texas. We live in an area where heavy rainfall can cause havoc and flooding is still a very real threat to our communities.” Other signs are planned in south and central Texas in early 2010. The National Weather Service Forecast Office in Shreveport will also work to plan additional installations over north Louisiana next year. The signs not only document major historical floods, but more importantly convey the potential impact that floods have had and could have again on communities. Floods are one of the most deadly and most costly natural disasters, and more people lose their lives in floods than due to lightning or tornadoes. The signs are a good reminder to maintain our vigilance and evaluate our preparedness for floods. The peak of Jefferson’s record flood occurred on April 2, 1945 when the Big Cypress Bayou rose to a stage of 30 feet, inundating parts of the town with more than five feet of water. This flood led to the United States Congress passing the Flood Control Act of 1946. This piece of legislation eventually established the many flood control reservoirs in the Red River Valley across Northeast Texas, including Lake O’ the Pines a few miles upstream of Jefferson. The flood also caused the city to abandon the developed areas on the south bank of the river. The National Weather Service office in Shreveport, the Lower Mississippi River Forecast Center (LMRFC), the Marion County Historical Commission, and Jefferson City Officials cooperated on this project. The LMRFC creates forecast guidance of river levels and flows for over 200 points in the southern U.S. including the Big Cypress Bayou at Jefferson. The National Weather Service used US Army Corps of Engineers and US Geological Survey data since they maintain a real-time hydrologic monitoring network of stage, streamflow, and rainfall gages that are a crucial component of the NWS’s river forecast and flood warning programs.

Another cut off low has formed across the country this week. This one is a cold core low bringing rain and even snow to parts of the High Plains and Rockies. Ahead of this low is a cold front that is slowly moving through Texas this morning. Cooler slightly drier air will move in behind this front but, we will still see a chance of showers in East Texas thanks to weak disturbances swinging around the upper low to our northwest.

Temperatures this morning across the great state of Texas ranged from 34°F in Burleson to 85° in Port Aransas. Temperatures in East Texas will drop into the lower 60s tonight with the possibility of seeing a few upper 50s in our western counties, closer to the much colder air that will spread across West Texas over night.

Monday, September 14, 2009

One year ago yesterday at 2:10AM CDT, hurricane Ike made landfall on Galveston Island. Ike was a strong category 2 storm that covered a tremendous amount of area in the Gulf of Mexico. Because of this, Ike produced a catastrophic storm surge one would expect from a Category 4 or higher storm. Ike will probably go down as the third costliest natural disaster in U.S. history behind Hurricanes Katrina and Andrew. Ike began as a tropical wave off the African Coast and became a tropical depression on the morning of September 1st. Later that same afternoon Ike strengthened into a tropical storm. The rapid intensification of Ike continued over the next 72 hours with Ike reaching a category 4 status by the evening of the 4th with top winds of 145mph. On September 7th Ike hit the island of Great Inagua near the Curks and Caicos followed by western Cuba that evening. The higher terrain of Cuba helped weaken Ike to a top wind speed of 100 mph as he entered the Gulf of Mexico. However, the amount of real estate Ike possessed was tremendous. Just look at the size of Ike when the center of the storm was a few hundred miles south of Galveston.He nearly takes up the entire Gulf of Mexico. It was Ike’s large area that enabled it produce such a large storm surge as water was funneled into the shallow waters of the Galveston Bay and Sabine Lake in the Northwest Gulf of Mexico. According to the National Hurricane Center the highest water mark from Ike’s storm surge was 17.5’ above mean sea level 10 miles inland across Chambers County. 20 lives were lost across the Gulf coast from Ike and another five lives were lost in Arkansas due to high winds. Right now the official damage total is around $19 billon making Ike the fourth costliest storm in U.S. history but once all insurance claims are taken care of, Ike will more than likely move up to number three on the list. <

Tuesday, September 8, 2009

On the evening of September 8th, 1900, Galveston Texas was struck by a category 4 hurricane estimated wind speeds of up to 135 mph and a storm surge of 15 to 20 feet. After the storm had past, up to 8000 lives were lost and the coast line was changed forever.

The origin of the “Great Hurricane” remains unclear due to the limited observation ability 109 years ago. It is believed the storm originated as many powerful Atlantic Basin hurricanes do off the west coast of Africa. On August 27th an area of “unsettled weather” 1000 miles east of the Windward Islands was reported by a ship (source Neil L. Frank). This area of disturbed weather developed into a tropical storm and moved over the Island of Cuba and entered the Florida Straits as either a strong tropical storm or a weak hurricane on September 5th. The storm continued to move west northwest until making landfall on Galveston Island on the night of the 8th. The destruction was unbelievable. For more information on this catastrophic storm click the following photos:

Tuesday, September 1, 2009

Sunday evening a rapidly developing thunderstorm developed off the Galveston Coast. The storm developed over two intersecting outflow boundaries from earlier storms. The result was a waterspout which moved on shore to become an EF-1 tornado. Here is the official report from the National Weather Service in Houston:...NWS STORM SURVEY RESULTS FOR GALVESTON TORNADO...

AT AROUND 948 PM ON SUNDAY...AUGUST 30TH...A WATERSPOUT MOVEDONSHORE ON GALVESTON ISLAND NEAR THE INTERSECTION OF 29TH STREET ANDSEAWALL BLVD. THE TORNADO THEN PRODUCED AN INTERMITTENT DAMAGE PATHFOR ABOUT ONE THIRD OF A MILE TO THE NORTHWEST...ENDING NEAR THEINTERSECTION OF 31ST STREET AND AVENUE R. SEVERAL STRUCTURES WEREDAMAGED INCLUDING THE DOLPHIN WORLD SOUVENIR SHOP...WHERE WINDOWSWERE SHATTERED AND MOST OF THE METAL ROOF WAS PEELED OFF. AGALVESTON POLICE DEPARTMENT PATROL CAR HAD ITS WINDOWS BLOWN OUT INTHE SAME AREA. HEADING FURTHER INLAND...THE TORNADO PUSHED OVER AHOME ON STILTS AND REMOVED MOST OF THE ROOF OF A MOBILE HOME.ANOTHER HOME AT THE INTERSECTION OF 31ST STREET AND AVENUE Q 1/2 HADITS ROOF COMPLETELY REMOVED. BEYOND THIS LOCATION DAMAGE WAS LIMITEDTO SMALL TREE LIMBS AND SOME SHINGLES REMOVED FROM ROOFS.

THE OBSERVED DAMAGE SUGGESTS WIND SPEEDS IN THE 80 TO 90 MPHRANGE...WITH MAXIMUM WINDS LEADING TO AN EF-1 RATING ON THE ENHANCEDFUJITA SCALE. DAMAGE PATH LENGTH WAS AROUND ONE THIRD OF AMILE...WITH A WIDTH OF AROUND 100 FEET. THERE WERE THREE INJURIESREPORTED. TWO POLICE OFFICERS...WHO WERE IN THE PATROL CAR WHEN ITSWINDOWS SHATTERED...WERE TREATED AND RELEASED. ANOTHER MAN...WHO WASTHROWN AGAINST A WALL...REMAINS HOSPITALIZED AT UTMB.Most waterspouts form under developing cumulus towers along sea breeze and land breeze fronts where the surface vorticity can be stretched in the vertical under the developing cloud updrafts. These so called “fair weather spouts” are usually short lived and remain off shore. Every once in a while one of these spouts will make land fall becoming officially a tornado but rarely producing more than EF-0 damage. The water spout that formed Sunday night appears to have formed in the same way the Beaumont Tornado formed earlier in August. By looking at the radar data from Sunday night we can see that two outflow boundaries were interesting each other just off the coast of Galveston. At this intersection a new storm cell rapidly developed indicating an area of strong vertical growth. This rapid upward movement was able to stretch the surface vorticity located over the intersection allowing for the waterspout to form.

Thanks to the amount of low level energy available(08/31/09 OZ RUC 0-3km CAPE over 200 J/kg off shore) for storm development, a strong updraft was able to form translating to a stronger wind speed in the tornado as it moved on shore, producing EF-1 damage. Below are a couple of radar loops of the develop storm which spawned the tornado. In the first you can see the two boundaries with the cell rapidly developing over the intersection. In the second loop you can see the rapid vertical development showing signs of a strong updraft over the intersecting boundaries.